This invention relates to the removal of hydrogen sulfide from gas streams by contacting the hydrogen sulfide containing gas stream with an aqueous solution containing thiosulfate ions to form elemental sulfur with the subsequent regeneration of thiosulfate ion by the oxidation of hydrogen sulfide.
Hydrogen sulfide is often present in many gas streams as a contaminant which prevents the untreated discharge of gas streams to the atmosphere or the use of components of certain gas streams for other productive purposes. Over the years many desulfurization processes have been developed in attempts to produce gas streams substantially free of hydrogen sulfide. Many of these processes are operated in a cyclical manner and employ regenerable oxidizing agents to oxidize hydrogen sulfide to elemental sulfur which is recovered and to discharge to the atmosphere or reuse the gas stream. Regeneration of the reduced oxidizing agent in an aqueous absorbing medium, after its oxidation of the hydrogen sulfide to sulfur, is typically accomplished by contacting the aqueous medium with air or oxygen gas.
Many such processes have been attempted often using a relatively expensive organic material in the solution. A common detriment attributed by those skilled in the art with many of these so-called wet desulfurization processes, particularly those involving some organic derivatives such as quinones, is the formation of thiosulfate ions as a by-product. Much effort and creativity has been directed towards the reduction or elimination of thiosulfates or polythionates present in the oxidizing liquor used to absorb the hydrogen sulfide from the gas stream. Several such attempts are described for example, in U.S. Pat. No. 4,347,212. One Patent, U.S. Pat. No. 3,773,662, describes a process for removing thiosulfates from solutions by introducing hydrogen sulfide gas into the liquid stream to decompose the polysulfide and suppress hydrolysis of elemental sulfur during cooling thus increasing the recovery of sulfur from a sulfur plant. The presence of thiosulfate was treated as an undesirable nuisance.
It has long been recognized that the polythionate or thiosulfate salt in aqueous solution will react with hydrogen sulfide to form sulfur. For instance, U.S. Pat. No. 1,832,325 teaches a process for removing hydrogen sulfide by oxidizing sodium thiosulfate to sodium tetrathionate and reacting it with hydrogen sulfide. It also discloses the use of buffering salts such as sodium bicarbonate or acid sodium phosphates to keep the hydroxide ion concentration below a value above which sodium tetrathionate would become unstable. Oxidation of the thiosulfate to tetrathionate is accomplished by the use of certain catalyst of nickel or cobalt.
U.S. Pat. No. 1,937,196 teaches the removal of hydrogen sulfide from a gas stream by the contact of a sodium carbonate solution which is later regenerated by heating and aeration. This patent teaches the use of thiosulfate salts such as sodium thiosulfate, ammonium thiosulfate, zinc thiosulfate, iron thiosulfate and the like for the absorption of sulfur dioxide wherein the thiosulfate is regenerated by contact with hydrogen sulfide. Sulfur is recovered after such contact with the thiosulfate solution having absorbed sulfur dioxide.
In U.S. Pat. No. 3,536,618 there is described a sulfur removal step wherein ammonium thiosulfate is reacted with hydrogen sulfide in a gas stream which is then decomposed to recover ammonium, hydrogen sulfide, water and elemental sulfur. The resulting aqueous stream is recycled to remove sulfide salts from a hydrocarbon product stream by a simple absorption. This process does not deal with the clean-up of a gas stream but does disclose the reaction of a thiosulfate with hydrogen sulfide to form elemental sulfur.
Hydrogen sulfide removal in the presence of an activated carbon catalyst and stoichiometric amounts of oxygen is described in U.S. Pat. No. 4,263,271. There is no liquid phase involved in the discussion with respect to this patent.
The removal of hydrogen sulfide from a gas stream by contact with a regenerable washing solution having a pH of between about 5 and 10 and containing solubilized vanadium, thiocyanate ions, a carboxylate complexing agent, one or more water soluble quinones and one or more water soluble nonquinone aromatic compounds capable of solubilizing tetravalent vanadium is described in U.S. Pat. No. 4,432,962 which issued on Feb. 21, 1984. The absorbed hydrogen sulfide is converted to elemental sulfur and after oxidative regeneration of the washing solution is separated from the regenerated solution.
U.S. Pat. No. 4,243,648 also describes a hydrogen sulfide removal and conversion method wherein a hydrogen sulfide containing gas stream is contacted with a regenerable washing solution capable of absorbing hydrogen sulfide and converting the hydrogen sulfide to hydrophobic elemental sulfur particles. After regeneration of the washing solution having a pH of between 5 and 10 which contains solubilized vanadium, thiocyanate ions, a solubilizing agent for tetravalent vanadium and a water soluble carboxylate complexing agent, the elemental sulfur form is separated. The patent describes this process as being particularly useful for removal of hydrogen sulfide from gas streams produced by the sweetening of sour natural gas, processing of ores, the destructive distillation of coal and oil shale, the gasification or liquifaction of coal, the use of geothermal fluids to generate electricity or other processes which produce hydrogen sulfide containing gases.
Notwithstanding the disclosures of the foregoing prior art, much is still to be accomplished in the removal of hydrogen sulfide from contaminated gas streams. Previously hydrogen sulfide was removable only by employing multiple unit operations such as, for example, an amine scrubber followed by a Claus unit which in turn was followed by a tail gas treatment and still sulfur was left in the gas stream. To accomplish a technical solution to the problem where removal of hydrogen sulfide and sulfur recovery is simply and efficiently accomplished the invention herein described was made. Many advantages of the present invention will become manifest in view of the following description.